Li-rich layered oxides have received the spotlight as cathode materials to improve the energy density in recent years. However, Li-rich layered oxides accompanied by cation migration during extended cycles suffer from low-capacity retention and structural degradation through the phase transition. In this study, we synthesized a Li2IrO3 material substituting Sn for Ir, confirming that Li2Ir0.75Sn0.25O3 exhibits improved cycle performance and structural stability. This enhancement is due to the highly reversible structural changes originating from the biphasic reaction, including the O3′ phase. The intermediate O3′ phase has a distorted IrO6 octahedron by the migration of Sn, thus enlarging interslab thickness and providing a facile Li diffusion environment. More importantly, migrated Sn ions can return to the transition metal layer during the discharging process. This reversible cation migration prevents structural collapse, thus improving cycle performance. These fundamental understandings of reversible cation migration for the Li-rich materials can provide insightful factors for designing high-energy cathode materials.